This is usually due to one of three things:
1)The yarn you're using is too soft. All of the tufting machines prefer courser yarns.
2) The machine has been adjusted too much, so the scissors aren't working. To test (with machine off) rotate the white gear with your hand 360 degrees. You should see the pair of scissors come out of the end of the needle, create a scissor action, then retreat back into the machine. It's also important to check the location of the bolt that is attached to the white gear. This can be misaligned if the pile height is too high. See image below for correct positioning. If this doesn't work you'll need to lower the pile height until they work again.
3) The metal bar at the front of the machine needs to be adjusted to push the scissor blade together. The scissor action is a mechanical process meaning when the open scissors are going back and forth they hit a metal piece at the front of the machine, which causes them to close. If the scissors aren't working you'll need to use a small alen wrench to move the metal bar closer to the back of the machine. It's usually less than 1/8" (3mm). To test simply move the metal piece and rotate the gear by hand. Continue making adjustments to the metal bar until the action is smooth. See video below for help.
If these two things don't solve the issue it may need more detailed adjustments. Reach out and we'll help.